1. Going hand in hand with viral infection in the body is the immune response the body mounts to counter it.

2. There are a number of terms we must consider in thinking about the immune system. They include the following

Antibody - a protein made by the immune system that will bind to a specific structure found in an antigen

Antigen - a molecule (usually a protein) recognized by an antigen. An antigen contains a specific structure within it called an epitope.

Epitope - a specific structure of an antigen bound by an antibody

B-cells - cells of the immune system that make antibodies and are involved in the humoral response

T-cells - cells of the immune system that can kill invading cells or infected cells. Involved in the cellular response

Humoral response - a response of the immune system that involves secretion of antibodies produced in the B-cells (also called B-lymphocytes)

Cellular response - an immune system response that involves T-cells, but does not involve antibodies. Special killer cells are involved in this response.

3. The structure of antibodies is that of a Y-shaped molecule. The Y contains two longer protein chains (called heavy chains) and two shorter protein chains (called light chains). The chains are held together in the Y shape by disulfide bonds.

4. The heavy and light chains have regions that are relatively unchanged from one antibody to another (called constant regions) and other regions that vary significantly from one antibody to another (called variable regions).

5. The variable regions of the antibody are found at the top of the Y and are the parts that bind to the antigen.

6. When B-cells bind to an antigen with their antibody, they can be stimulated to divide. A B-cell that is stimulated to divide replicates exactly the antibody that recognized the antigen. Thus, binding of one antigen by a B-cell antibody can stimulate production of millions of identical antibodies.

7. Antibodies are useful as research tools. They are used as follows - imagine you are studying a specific protein and want to make an antibody against it. You inject the protein into an animal (rabbit, for example). The rabbit's immune system recognizes the new protein and foreign and the immune system makes millions of antibodies against it.

8. One can take blood from the rabbit with the antibodies in it and easily purify the antibodies away from all of the other blood components. The antibody can then be 'tagged' with a fluorescent dye, for example. Then that tagged antibody can be used to show if the antigen it recognizes (the original protein of interest) is present in samples or not. One way to do this is to bind the material in several samples to a membrane (or filter paper). Then the tagged antibodies are added, allowed to find antigens, and then unbound antibodies are washed away. Wherever the fluorescent dye is found indicates which samples had the antigen.

9. T-cells can help to stimulate B-cells to divide. They can also recognize when a host cell is infected with a virus or is having a problem (including becoming cancerous). When a killer T-cell recognizes such a cell, it can do several things. One, it can secrete a protein (called perforin) that drills a hole in the cell wall of the cell and cause it to die. Second, it can induce the damaged cell to commit suicide. This phenomenon is called "apoptosis".